Compounds with mixed anions as solid Li-ion conductors

ABSTRACT

A solid-state lithium ion electrolyte is provided which contains a composite material having at least 94 mole % lithium ions as cation component and multiple anions in an anionic framework capable of conducting lithium ions. An activation energy for lithium ion migration in the solid state lithium ion electrolyte is 0.5 eV or less. Composites of specific formulae are provided. A lithium battery containing the composite lithium ion electrolyte is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior U.S. applicationSer. No. 15/805,672, filed Nov. 7, 2017, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND

Li-ion batteries have traditionally dominated the market of portableelectronic devices. However, conventional Li-ion batteries containflammable organic solvents as components of the electrolyte and thisflammability is the basis of a safety risk which is of concern and couldlimit or prevent the use of Li-ion batteries for application in largescale energy storage.

Replacing the flammable organic liquid electrolyte with a solidLi-conductive phase would alleviate this safety issue, and may provideadditional advantages such as improved mechanical and thermal stability.A primary function of the solid Li-conductive phase, usually calledsolid Li-ion conductor or solid state electrolyte, is to conduct Li ionsfrom the anode side to the cathode side during discharge and from thecathode side to the anode side during charge while blocking the directtransport of electrons between electrodes within the battery.

Moreover, lithium batteries constructed with nonaqueous electrolytes areknown to form dendritic lithium metal structures projecting from theanode to the cathode over repeated discharge and charge cycles. If andwhen such a dendrite structure projects to the cathode and shorts thebattery energy is rapidly released and may initiate ignition of theorganic solvent.

Therefore, there is much interest and effort focused on the discovery ofnew solid Li-ion conducting materials which would lead to an all solidstate lithium battery. Studies in the past decades have focused mainlyon ionically conducting oxides such as for example, LISICON(Li₁₄ZnGe₄O₁₆), NASICON(Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃), perovskite (forexample, La_(0.5)Li_(0.5)TiO₃), garnet (Li₇La₃Zr₂O₁₂), LiPON (forexample, Li_(2.88)PO_(3.73)N_(0.14)) and sulfides, such as, for example,Li₃PS₄, Li₇P₃S₁₁ and LGPS (Li₁₀GeP₂S₁₂).

While recent developments have marked the conductivity of solid Li-ionconductor to the level of 1-10 mS/cm, which is comparable to that inliquid phase electrolyte, finding new Li-ion solid state conductors isof great interest.

An effective lithium ion solid-state conductor will have high Li⁺conductivity at room temperature and low activation energy of Limigration in the conductor for use over a range of operationtemperatures that might be encountered in the environment. Further,unlike many conventionally employed non-aqueous solvents, thesolid-state conductor material should be stable to electrochemicaldegradation reactivity with the anode and cathode chemical composition.Environmentally stable composite materials having high Li⁺ conductivityand low activation energy would be sought in order to facilitatemanufacturing methods and structure of the battery.

The standard redox potential of Li/Li+ is −3.04 V, making lithium metalone of the strongest reducing agent available. Consequently, Li metalcan reduce most known cationic species to a lower oxidation state.Because of this strong reducing capability when the lithium metal of ananode contacts a solid-state Li⁺ conductor containing cation componentsdifferent from lithium ion, the lithium reduces the cation specie to alower oxidation state and deteriorates the solid-state conductor.

For example, the conductor of formula:Li₃PS₄contains P⁵⁺ in the formula and is thus a secondary cation to the Li⁺.When in contact with Li metal Li, reduction according to the followingequation occurs (J. Mater. Chem. A, 2016, 4, 3253-3266).Li₃PS₄+5Li→P+4Li₂SP+3Li→Li₃P

Similarly, Li₁₀GeP₂S₁₂ has also been reported to undergo degradationwhen in contact with lithium metal according to the following equations(J. Mater. Chem. A, 2016, 4, 3253-3266):Li₁₀GeP₂S₁₂+10Li→2P+8Li₂S+Li₄GeS₄P+3Li→Li₃P4Li₄GeS₄+31Li→16Li₂S+Li₁₅Ge₄Li₁₀GeP₂S₁₂ contains Ge⁴⁺ and P⁵⁺ and each is reduced as indicated.

In another example, Li₇La₃Zr₂O₁₂, which contains secondary cations La³⁺and Zr⁴⁺ undergoes chemical degradation when in contact with lithiummetal according to the following chemistry (J. Mater. Chem. A, 2016, 4,3253-3266):6Li₇La₃Zr₂O₁₂+40Li→4Zr₃O+41Li₂O+9La₂O₃Zr₃O+2Li→Li₂O+3ZrLa₂O₃+6Li→2La+3Li₂O

Thus, many current conventionally known solid Li-ion conductors suffer astability issue when in contact with a Li metal anode.

The inventors of this application have been studying lithium compositecompounds which may serve for future use of solid-state Li+ conductorsand previous results of this study are disclosed in U.S. applicationSer. No. 15/626,696, filed Jun. 19, 2017. However, composites of highestefficiency, highest stability, low cost and ease of handling andmanufacture continue to be sought.

Accordingly, an object of this application is to identify a range offurther materials having high Li ion conductivity while being poorelectron conductors which are suitable as a solid state electrolyte fora lithium ion battery.

A further object of this application is to provide a solid state lithiumion battery containing a solid state Li ion electrolyte membrane.

SUMMARY OF THE EMBODIMENTS

These and other objects are provided by the embodiments of the presentapplication, the first embodiment of which includes a solid-statelithium ion electrolyte, comprising: a composite material having atleast 94 mole % lithium ions as cation component and multiple anions; inan anionic framework capable of conducting lithium ions; wherein anactivation energy for lithium ion migration in the solid state lithiumion electrolyte is 0.5 eV or less.

In an aspect of the first embodiment a lithium ion (Li) conductivity ofthe solid state lithium ion electrolyte is at least 10⁻⁶ S/cm at roomtemperature.

One aspect of the first embodiment includes a solid-state lithium ionelectrolyte of formula (I):Li_(7-nx)M_(x)Br₃O₂  (I)

wherein

M is cation of n+ charge,

x is a number from 0 to 0.7, and

n is 2 or 3.

A second aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (II):Li_(10-ny)M_(y)N₃Br  (II)

wherein

M is cation of n+ charge,

y is a number from 0 to 1.0, and

n is 2 or 3.

A third aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (III):Li_(5-nz)M_(z)NCl₂  (III)

wherein M is cation of n+ charge,

z is a number from 0 to 0.5, and

n is 2 or 3.

A fourth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (IV):Li_(4-nw)M_(w)NCl  (IV)

wherein M is cation of n+ charge,

w is a number from 0 to 0.4, and

n is 2 or 3.

A fifth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (V):Li_(6-nt)M_(t)NBr₃  (V)

wherein M is cation of n+ charge,

t is a number from 0 to 0.6, and

n is 2 or 3.

A sixth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (VI):Li_(6-ns)M_(s)NI₃  (VI)

wherein M is cation of n+ charge,

x is a number from 0 to 0.6, and

n is 2 or 3.

In a second embodiment a solid state lithium battery is included. Thesolid state lithium battery comprises: an anode; a cathode; and a solidstate lithium ion electrolyte located between the anode and the cathode;wherein the solid state lithium ion electrolyte comprises a compositematerial according to any of the aspects of the first embodiment.

The forgoing description is intended to provide a general introductionand summary of the present disclosure and is not intended to be limitingin its disclosure unless otherwise explicitly stated. The presentlypreferred embodiments, together with further advantages, will be bestunderstood by reference to the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows diffusivity of Li obtained from ab initio moleculardynamics simulation for selected composites of aspects of the firstembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout this description, the terms “electrochemical cell” and“battery” may be employed interchangeably unless the context of thedescription clearly distinguishes an electrochemical cell from abattery. Further the terms “solid-state electrolyte” and “solid-stateion conductor” may be employed interchangeably unless explicitlyspecified differently.

Structural characteristics of effective Li⁺ conducting crystal latticeshave been described by Ceder et al. (Nature Materials, 14, 2015,1026-1031) in regard to known Li⁺ ion conductors Li₁₀GeP₂S₁₂ andLi₇P₃S₁₁, where the sulfur sublattice of both materials was shown tovery closely match a bcc lattice structure. Further, Li⁺ ion hoppingacross adjacent tetrahedral coordinated Li⁺ lattice sites was indicatedto offer a path of lowest activation energy.

The inventors are investigating new lithium composite compounds in orderto identify materials having the properties described above which mayserve as solid-state electrolytes in solid state lithium batteries. Inthe course of this ongoing study and effort the inventors have developedand implemented a methodology to identify composite materials which havechemical and structural properties which have been determined by theinventors as indicators of lithium ion conductance suitable to be asolid state electrolyte for a lithium-ion battery.

As described above, the inventors have recognized that a root cause ofthe instability of a lithium composite compound against a Li metal anodeis the reduction of non-lithium cations (secondary cations) in thecompounds. Accordingly in the developed methodology, (a) only compositelithium compounds having no secondary cation(s) may be considered. Inthis regard, materials containing only Li as cation specie may beincluded. In theory, any anion which meets the other criteriarequirements described herein may be present in the compositestructures. One of knowledge and skill in lithium composite chemistrywill be able to identify anions suitable for study according to themethod disclosed in this application.

If an anion containing at least one of N³⁻, O²⁻, S²⁻, F⁻, Cl⁻, Br⁻ andI⁻ is selected as a basis for evaluation, seven compounds of formulaeLi₃N, Li₂O, Li₂S, LiF, LiCl, LiBr and LiI may be considered.

A criterion of this methodology requires that to qualify as solid stateelectrolyte in practical application, the material must exhibitdesirable Li-ion conductivity, usually no less than 10⁻⁶ S/cm at roomtemperature. Thus, ab initio molecular dynamics simulation studies arewere applied to calculate the diffusivity of Li in the latticestructures of these seven materials. In order to accelerate thesimulation, the calculation was performed at high temperatures and theeffect of excess Li or Li vacancy was considered. In order to createexcess Li or Li vacancy, aliovalent replacement of cation or anions wasevaluated. Thus, Li vacancy was created by, for example, partiallysubstituting N³⁻ anion for the anions of the material. Li vacancies mayalso be created by partially replacing Li cation with higher valentcations such as Mg⁺². The diffusivity at 300 K was determined accordingto equation (I)D=D ₀ exp(−E _(a) /k _(b) T)  equation (I)where D₀, E_(a) and k_(b) are the pre-exponential factor, activationenergy and Boltzmann constant, respectively. The conductivity is relatedwith the calculated diffusivity according to equation (II):σ=D ₃₀₀ ρe ² /k _(b) T  equation (II)where ρ is the volumetric density of Li ion and e is the unit charge.

The only compound with room temperature Li-ion conductivity higher than10⁻⁶ S/cm was determined to be Li₃N, while the remaining compounds haveconductivity lower than 10⁻¹⁰ S/cm.

Accordingly, the inventors adapted the methodology to consider (b)compounds with two or more types of anions, defined as mixed anions, inthe formula. Combining condition (a) and (b), the methodology limits theLi-ion conductors according to the first embodiment to compounds (a) nothaving a secondary cation and (b) to compounds having at least twodifferent (mixed) anions.

The anionic lattice of Li-ion conductors has been shown to match certainlattice types (see Nature Materials, 14, 2015, 2016). Therefore, in (c)the anionic lattice of the potential Li⁺ ion conductor is compared tothe anionic lattice of Li⁺ ion conductor known to have highconductivity.

Thus, lithium compounds (a) having no secondary cation and (b) havingmixed anions of N³⁻, O²⁻, S²⁻, F⁻, Cl⁻, Br⁻ and I⁻ were compared toLi-containing compounds reported in the inorganic crystal structuredatabase (FIZ Karlsruhe ICSD—https://icsd.fiz-karlsruhe.de) andevaluated in comparison according to an anionic lattice matching methoddeveloped by the inventors for this purpose and described in copendingU.S. application Ser. No. 15/597,651, filed May 17, 2017, to match thelattice of these compounds to known Li-ion conductors.

According to the anionic lattice matching method described in copendingU.S. application Ser. No. 15/597,651, an atomic coordinate set for thecompound lattice structure may be converted to a coordinate set for onlythe anion lattice. The anions of the lattice are substituted with theanion of the comparison material and the obtained unit cell rescaled.The x-ray diffraction data for modified anion-only lattice may besimulated and an n×2 matrix generated from the simulated diffractiondata. Quantitative structural similarity values can be derived from then×2 matrices.

The purpose of anionic lattice matching is to further identify compoundswith greatest potential to exhibit high Li⁺ conductivity. From thiswork, the compounds listed in Table 1 were determined to be potentiallysuitable as a solid-state Li⁺ conductor. Among the compounds identified,Li₃OBr was reported as a solid Li conductor (Zhao et al., J Am. Chem.Soc., 2012, 134, 15042).

TABLE 1 Compounds meeting requirement (a) and (b) with the latticematching to known solid Li-ion conductors. Compound Li₇Br₃O₂ Li₁₀N₃BrLi₅NCl₂ Li₄NCl Li₆NBr₃ Li₆NI₃ Anions Br⁻, Br⁻, Cl⁻, Cl⁻, Br⁻, I⁻, O²⁻N³⁻ N³⁻ N³⁻ N³⁻ N³⁻ Lattice LiZnPS₄ Li₁₀GeP₂S₁₂ Li₆PS₅Br Li₃N Li₃OBrLi₃OBr matched to

Ab initio molecular dynamics (AIMD) simulation was then applied topredict the conductivity of Li₄NCl, Li₇Br₃O₂ and Li₁₀N₃Br. Thesimulation was carried out with a small amount of Mg replacing Li tocreate the mobile Li vacancy. To accelerate the simulation, thecalculation was performed at high temperatures. The FIGURE shows thecalculated diffusivity for each of the three studied compositions. Inthe temperature range of 800-1650 K, the diffusivity for all compoundsare in the order of 10⁻⁴ to 10⁻⁶ cm²/s, and shows good Arrheniusdependence on temperature.

Table 2 lists the activation energy barriers and the conductivities at300 K for these compounds. All three compounds have the conductivitiesabove 10⁻⁶ S/cm, meeting the requirement of solid Li-ion conductor. Moreimportantly, these compounds are stable when contacted with metal Li,indicating they can be used directly as solid state electrolyte withmetal Li anode.

TABLE 2 Activation energy and room temperature conductivity of Li₄NCl,Li₁₀N₃Br and Li₇Br₃O₂ from AIMD simulations. Compound Composition inAIMD simulation E_(a) (eV) σ (S/cm) Li₄NCl Li_(3.56)Mg_(0.22)NCl 0.262.1 × 10⁻³ Li₁₀N₃Br Li_(9.33)Mg_(0.33)N₃Br 0.45 7.9 × 10⁻⁶ Li₇Br₃O₂Li₆Mg_(0.5)Br₃O₂ 0.43 2.6 × 10⁻⁶

As described the compounds are doped by replacing a maximum of 10 mole %of a total cation content of lithium with a dopant cation having a +2 or+3 charge in order to create vacancies for lithium mobility, whilemaintaining charge neutrality.

Accordingly, in the first embodiment, the present application provides asolid-state lithium ion electrolyte, comprising: comprising: a compositematerial having at least 94 mole % lithium ions as cation component andmultiple anions; in an anionic framework capable of conducting lithiumions; wherein the composite material is in the form of an anionicframework capable of conducting lithium ions, and an activation energyfor lithium ion migration in the solid state lithium ion electrolyte is0.5 eV or less.

In an aspect of the first embodiment a lithium ion (Li⁺) conductivity ofthe solid state lithium ion electrolyte is at least 10⁻⁶ S/cm at roomtemperature.

One aspect of the first embodiment includes a solid-state lithium ionelectrolyte of formula (I):Li_(7-nx)M_(x)Br₃O₂  (I)

wherein

M is cation of n+ charge,

x is a number from 0 to 0.7, and

n is 2 or 3.

A second aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (II):Li_(10-ny)M_(y)N₃Br  (II)

wherein

M is cation of n+ charge,

y is a number from 0 to 1.0, and

n is 2 or 3.

A third aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (III):Li_(5-nz)M_(z)NCl₂  (III)

wherein M is cation of n+ charge,

z is a number from 0 to 0.5, and

n is 2 or 3.

A fourth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (IV):Li_(4-nw)M_(q)NCl  (IV)

wherein M is cation of n+ charge,

w is a number from 0 to 0.4, and

n is 2 or 3.

A fifth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (V):Li_(6-nt)M_(t)NBr₃  (V)

wherein M is cation of n+ charge,

t is a number from 0 to 0.6, and

n is 2 or 3.

A sixth aspect of the first embodiment includes a solid state lithiumion electrolyte of formula (VI):Li_(6-ns)M_(s)NI₃  (VI)

wherein M is cation of n+ charge,

s is a number from 0 to 0.6, and

n is 2 or 3.

Further to each of the six aspects a lithium ion (Li⁺) conductivity ofthe solid state lithium ion electrolyte may be at least 10⁻⁶ S/cm atroom temperature.

Synthesis of the composite materials of the first embodiment may beachieved by solid state reaction between stoichiometric amounts ofselected precursor materials.

For example, Li₄NCl can be synthesized from Li₃N and LiCl at 450° C.(Journal of Solid State Chemistry, 128, 1997, 241). Li₁₀N₃Br can beprepared from Li₃N and LiBr at 500° C. (Zeitschrift für NaturforschungB, 50, 1995, 1061). Li₅NCl₂ can be prepared from Li₃N and LiCl in asolid state reaction at 450° C. (Journal of Solid State Chemistry 130,1997, 90). Li₆NBr₃ can be prepared from Li₃N and LiBr at 430° C.(Journal of Alloys and Compounds, 645, 2015, S174). Li₆NI₃ can beprepared from Li₃N and LiI at 490° C. (Z. Naturforsch. 51b, 199652 5)

In further embodiments, the present application includes solid statelithium ion batteries containing the solid-state electrolytes describedabove. Solid-state batteries of these embodiments including metal-metalsolid-state batteries may have higher charge/discharge rate capabilityand higher power density than classical batteries and may have thepotential to provide high power and energy density.

Thus in further embodiments, solid-state batteries comprising: an anode;a cathode; and a solid state lithium ion electrolyte according to theembodiments described above, located between the anode and the cathodeare provided.

The anode may be any anode structure conventionally employed in alithium ion battery. Generally such materials are capable of insertionand extraction of Li⁺ ions. Example anode active materials may includegraphite, hard carbon, lithium titanate (LTO), a tin/cobalt alloy andsilicon/carbon composites. In one aspect the anode may comprise acurrent collector and a coating of a lithium ion active material on thecurrent collector. Standard current collector materials include but arenot limited to aluminum, copper, nickel, stainless steel, carbon, carbonpaper and carbon cloth. In an aspect advantageously arranged with thesolid-state lithium ion conductive materials described in the first andsecond embodiments, the anode may be lithium metal or a lithium metalalloy, optionally coated on a current collector. In one aspect, theanode may be a sheet of lithium metal serving both as active materialand current collector.

The cathode structure may be any conventionally employed in lithium ionbatteries, including but not limited to composite lithium metal oxidessuch as, for example, lithium cobalt oxide (LiCoO₂), lithium manganeseoxide (LiMn₂O₄), lithium iron phosphate (LiFePO₄) and lithium nickelmanganese cobalt oxide. Other active cathode materials may also includeelemental sulfur and metal sulfide composites. The cathode may alsoinclude a current collector such as copper, aluminum and stainlesssteel.

In one aspect, the active cathode material may be a transition metal,preferably, silver or copper. A cathode based on such transition metalmay not include a current collector.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. In thisregard, certain embodiments within the invention may not show everybenefit of the invention, considered broadly.

The invention claimed is:
 1. A solid-state lithium ion electrolyte,comprising: a composite material having lithium ions and a dopant cationM as cation component; and multiple anions; in an anionic frameworkcapable of conducting lithium ions; wherein the composite material is atleast one selected from the group of formulae consisting of (II), (III),(IV), (V) and (VI), a mole % content of the dopant cation is from 0 to10 mole % based on total moles of lithium and the dopant cation, and anactivation energy for lithium ion migration in the solid state lithiumion electrolyte is 0.5 eV or less,Li_(10-ny)M_(y)N₃Br  (II) wherein M is cation of n+ charge, y is anumber from 0 to 1.0, and n is 2 or 3;Li_(5-nz)M_(z)NCl₂  (III) wherein M is cation of n+ charge, z is anumber from 0 to 0.5, and n is 2 or 3,Li_(4-nw)M_(w)NCl  (IV) wherein M is cation of n+ charge, w is a numberfrom 0 to 0.4, and n is 2 or 3,Li_(6-nt)M_(t)NBr₃  (V) wherein M is cation of n+ charge, t is a numberfrom greater than 0 to 0.6, and n is 2 or 3 andLi_(6-ns)M_(s)NI₃  (VI) wherein M is cation of n+ charge, s is a numberfrom 0 to 0.6, and n is 2 or
 3. 2. The solid-state lithium electrolyteaccording to claim 1 wherein a lithium ion (Li⁺) conductivity of thesolid state lithium ion electrolyte is at least 10⁻⁶ S/cm at roomtemperature.
 3. A solid state lithium battery, comprising: an anode; acathode; and a solid state lithium ion electrolyte located between theanode and the cathode; wherein the solid state lithium ion electrolytecomprises the composite material of claim 1.